Ask An Expert is a question-and-answer column designed to address common questions related to forensic investigation and property damage. Each month we’ll feature one or two questions submitted by you – our readers and customers – and provide detailed, easy-to-understand answers. Email your questions to [email protected] or submit your questions here.
Question #1: With regards to boat docks, how do you differentiate between wind damage vs. water damage?
John Miller, P.E.: Wind speeds increase with height. Boat docks are typically the lowest point on the property. In Florida the boat docks are predominantly framed structures as opposed to floating structures. If the wind speeds were high enough to damage a wood-framed structure, we would expect significant damage to nearby roof coverings, siding, trees, etc. In most cases when we looked at boat docks, we had very little collateral evidence of wind. Secondly, if the damage is from water, then the components close to the water level are damaged while higher components such as a dock roof are not damaged. Finally, the weather and water data are critical in determining if sufficient wind or water heights existed to cause damage.
Question #2: Will wind result in storm surge if the sites are along the coast?
A: Moving water and waves with enough force to cause structural damage require wind to operate over open water for an extended period of time. The more open water and the longer time period that wind affects the area, the faster the water and the higher the waves. For example, Hurricane Sandy created a very large storm surge because it was an enormous storm (width) that traveled across open water for days. While there are no specific numbers you can use, you would not expect damaging water velocities and waves unless you had wind speeds greater than 20-30 mph operating over open water (several thousand feet at least) for an extended period of time (hours). Therefore, coastal properties are always more prone to storm surge.
Question #3: In areas where there were moderate winds we have seen zippering of roofing materials. I have generally seen this related to material or installation defect your thoughts on zippering?
A: We actually cover this in the aftershow on a case study. You can access the aftershow at https://donanuniversity.com/after-show/. Asphalt shingles become unsealed as they age. Unsealed shingles can lift in moderate winds and are more easily damaged. Moderate winds would be wind speeds that would not normally damage a properly installed and sealed shingle. When the shingles are installed in vertical columns (racking), the unsealed shingles are typically along the racking lines. This results in the zippered appearance of the damage. This also can result in a cascading failure initiated at the racking lines. The unsealed shingles lift and then lift overlapping shingles. In hurricanes, a building can be subjected to even moderate winds for hours making it possible for unsealed shingles to be damaged.
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About the Expert
About the Expert
John Miller joined Donan in 2009 and currently serves as a forensic engineer and principal consultant based out of the firm’s Louisville, Kentucky office. His area of expertise is structural engineering with an emphasis on structural damage from impact, wind and flood. Mr. Miller’s project capabilities also include a wide range of civil and structural investigations including hail damage, foundation damage and water intrusion. View John Miller’s full professional profile here.